The present invention generally relates to golf balls, and, in particular, to a composition of golf ball cores and covers comprising stable free radicals.
Solid core golf balls are well known in the art. Typically, the core is made from polybutadiene rubber material, which provides the primary source of resiliency for the golf ball. A known processing difficulty of polybutadiene cores cross-linked with peroxide initiators is the rapid increase in viscosity during cross-linking due to increase in polymer molecular weight. When the viscosity becomes too high, the partially cross-linked polymer cannot flow rapidly and does not completely fill the mold containing it. This phenomenon, known as scorch, results from premature cross-linking which may occur during compounding or manufacturing.
Scorch is accentuated by processing conditions that include high temperatures and/or high shear rates. For example, in an injection molding process, the elastomeric polymer and peroxide composition must be exposed to high shear rates and high temperatures to keep the composition fluid as it flows rapidly through the injection nozzle, runners and gates to the mold. If scorch occurs, the surface of the resulting molded golf ball core will be irregular and the composition may solidify in the runners leading to the mold, thereby unfavorably impacting the efficiency, scrap rate and safety of the process. High shear rates combined with high temperatures also occur in other common golf ball composition processing methods, such as in roll milling and extrusion.
A short scorch time (time between start of reaction and onset of crosslinking) further increases the occurrence of backrinding, the undesirable torn or gouged appearance of cross-linked articles at a mold parting line. Backrinding is caused by the continuing thermal expansion of the elastomeric polymer after cross-linking occurs, and is most severe for a spherical object, such as the outermost layers or portions of a golf ball core, where the surface area to mass ratio is at a minimum. Physical manifestations of backrinding are thought to contribute to premature failure of golf balls by making the golf ball less uniform, causing a focal stress at the gouge, and initiating cracks. Thus, it is desirable to reduce backrinding by increasing the scorch time in the cores to enhance golf ball durability.
One way to increase the scorch time is to decrease the decomposition rate of the peroxide initiator by lowering reaction temperature, resulting in a long half-life for the peroxide initiator. However, this approach is generally unsatisfactory, since a long half-life tends to result in a slow cross-linking rate and a long cure time. Recently, so-called scorch retarding or scorch resistant peroxides have become available. Exemplary applications of scorch retarding peroxides in golf balls are described in U.S. Pat. No. 6,339,119, the disclosure of which is incorporated herein by reference in its entirety. The scorch retarding peroxides slow the onset of scorch through free radical xe2x80x9cscavengersxe2x80x9d that react with the peroxide radicals and prevent them from immediately initiating cross-linking. However, these peroxides are only available in pre-mixed packages, which lack the flexibility of controlling the peroxide and the scavenger levels independently. Furthermore, existing systems, such as Varox(copyright) 802-40KE-HP available from R. T. Vanderbilt, do not significantly alter the cure state, unlike the compositions disclosed in the present invention.
Stable free radicals such as Tempo(copyright) have long been known to moderate the state of cure and extend the scorch time of elastomers by ways of reversibly quenching, scavenging and/or trapping early unstable free radicals formed during decomposition. Stable free radicals may further serve as a free radical capping additive or an antioxidant to prevent reaction of undesirable free radicals generated upon wear to the base rubber. It is therefore advantageous to incorporate stable free radicals in golf ball compositions, particularly in core and intermediate layers, in order to prolong scorch time for ease and flexibility of processing, as well as to impart durability and resilience to golf balls.
The present invention is directed to a golf ball having a core and a cover, particularly to a golf ball core formed from a composition including an elastomeric polymer, a free radical initiator, and at least one stable free radical. The stable free radical is present in an amount sufficient to increase the scorch time of the elastomeric polymer during curing by at least about 10%, preferably by at least about 25%, to substantially prevent backrinding. Preferably, the amount of the stable free radical is also sufficient to decrease the maximum torque of the elastomeric polymer during curing by at least about 5%, more preferably by at least about 10%. Specifically, the amount of the stable free radical is preferably between about 0.01 phr and about 20 phr by weight of the elastomeric polymer, more preferably between about 0.05 phr and about 5 phr, and most preferably between about 0.05 phr and about 1 phr.
Suitable stable free radicals for the present invention include, without limitation, nitroxide radicals; hydrazyl radicals; allyloxyl radicals; trityl radicals; and derivatives thereof having at least one substituent group comprising amino, isocyanate, hydroxyl, carboxyl, oxirane, thiirane; and mixtures thereof. Specific nitroxide radicals include, but are not limited to, 2,2,6,6-tetramethlpiperidinyloxy and derivatives thereof; 2,2,5,5-tetramethyl-1-pyrrolidinyloxy and derivatives thereof; 4,4-dimethyl-3-oxazolinyloxy and derivatives thereof; 2,6-di-t-butyl-xcex1-(3,5-di-t-butyl-4-oxo-2,5-cyclohexadien-1-ylidene)-p-tolyloxy and derivatives thereof; di-t-butyl nitroxide and derivatives thereof; and mixtures thereof. The most preferred choice of the stable free radical for the present invention is 2,2,6,6-tetramethlpiperidinyloxy or a derivative thereof.
The elastomeric polymer for the golf ball core of the invention can be one or more natural or synthetic elastomers, including without limitation natural rubbers; balata; gutta-percha; synthetic polyisoprenes; styrene-butadiene rubbers; styrene-propylene-diene rubbers; chloroprene rubbers; acrylonitrile rubbers; acrylonitrile-butadiene rubbers; ethylene-propylene-diene terpolymers; polypropylene resins; ionomer resins; polyamides; polyesters; urethanes; polyureas; thermosetting or thermoplastic elastomers; metallocene catalyzed rubbers; styrene-ethylene block copolymers; maleic anhydride or succinate modified metallocene catalyzed ethylene copolymers; chlorinated polyethylenes; polysulfide rubbers; flurocarbons; and mixtures thereof. Preferably, the elastomeric polymer comprises at least about 40 phr by weight of a polybutadiene having a cis-1,4 content of at least about 40%, a Mooney viscosity of at least about 20, a number molecular weight of at least about 150,000, and a polydispersity of less than about 4.0.
The free radical initiator for the golf ball core may be one or more peroxides; sulfur curing agents; high-energy radiation sources capable of generating free radicals; and mixtures thereof. Suitable peroxides include, but are not limited to, dicumyl peroxide; n-butyl-4,4-di(t-butylperoxy)-valerate; 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane; xcex1,xcex1xe2x80x2-bis(t-butylperoxy)-diisopropylbenzene; 2,5-dimethyl-2,5-di(t-butylperoxy)hexane; di-t-butyl peroxide; di-t-amyl peroxide; di(2-t-butyl-peroxyisopropyl)benzene peroxide; lauryl peroxide; benzoyl peroxide; t-butyl hydroperoxide; and mixtures thereof. Suitable sulfur curing agents include, without limitation, N-oxydiethylene 2-benzothiazole sulfenamide; N,N-di-orthotolyguanidine; bismuth dimethyldithiocarbamate; N-cyclohexyl-2-benzothiazole sulfenamide; N,N-diphenylguanidine; and mixtures thereof. And suitable high-energy radiation sources include electron beams; ultra-violet radiation; gamma radiation; X-ray radiation; infrared radiation; heat; and combinations thereof. The free radical initiator may further have an accelerator, such as mercaptobenzothiazole; sulfenamide; dithiocarbamate; thiuram sulfide; guanidine; thiourea; xanthate; dithiophosphate; aldehyde-amine; dibenzothiadyl disulfide; N,Nxe2x80x2-diccyclohexyl-2-benzothia-dylsulfenamide; hexamethylenetetramine; tetramethylthiuram disulfide; tetraethylthiuram disulfide; tetrabutylthiuram disulfide; and mixtures thereof. Preferably, the free radical initiator is dicumyl peroxide having an activity between about 70% and about 100%, and is present in an amount between about 0.05 phr and about 15 phr by weight of the elastomeric polymer.
The cross-linking agent for the core composition comprises salts of unsaturated carboxylic acids having about 3 to about 8 carbon atoms; unsaturated vinyl compounds; polyfunctional monomers; phenylene bismaleimide; sulfur; and mixtures thereof. Preferably, the crosslinking agent is zinc diacrylate having an activity between about 70% and about 100%, and is present in an amount less than about 40 phr by weight of the elastomeric polymer.
The core composition of the present invention can further include one or more free radical scavangers and scorch retarders to increase the scorch time of the elastomeric polymer. Suitable free radical scavangers include benzoquinones; hydroquinones; phenols; benzaldehydes; cyclohexadiene-1-ones; and mixtures thereof. Suitable scorch retarders include cyclohexylthiophthalimide; phthalic anhydride; pyromellitic anhydride; benzene hexacarboxylic trianhydride; 4-methylphthalic anhydride; trimellitic anhydride; 4-chlorophthalic anhydride; salicylic acid; benzoic acid; maleic anhydride; citraconic anhydride; itaconic anhydride; N-nitrosodiphenyl-amine; [1,3,5-tris(4-tert-butyl-3-hydroxy-2,6-dimethylbenzyl)-1,3,5-triazine-2,4,6-(1H,3H,5H)-trione]; mono-phenyl and bis-phenyl derivatives of allyl compounds; hydroquinones with optional sulfur accelerators; acid salts of aminoalkyl phenols; triazoles; thiomorpholines; monomeric monofunctional vinyl compounds; and mixtures thereof.
Other conventional additives for the core composition of the invention include cis-to-trans catalysts; fillers; foaming agents; antioxidants; processing aids, processing oils; plasticizers; dyes and pigments; and mixtures thereof. The golf ball core may be solid or fluid-filled, be a unitary mass, or have a center and at least one outer-core layer. The abovementioned stable free radical can be incorporated into any portion of the golf ball core, including the center, the outer core layer, or both.
The present invention is also directed to a golf ball cover formed of a composition that includes a stable free radical or a derivative thereof and at least one thermoplastic or thermosetting material. Preferably, the derivative of the stable free radical is a cationic N-oxoammonium salt formed from oxidation of a nitroxide radical that include, without limitation, 2,2,6,6-tetramethlpiperidinyloxy and derivatives thereof; 2,2,5,5-tetramethyl-1-pyrrolidinyloxy and derivatives thereof; 4,4-dimethyl-3-oxazolinyloxy and derivatives thereof; 2,6-di-t-butyl-xcex1-(3,5-di-t-butyl-4-oxo-2,5-cyclohexadien-1-ylidene)-p-tolyloxy and derivatives thereof; di-t-butyl nitroxide and derivatives thereof; and mixtures thereof. Suitable thermoplastic or thermosetting material for the cover can be homopolymers or copolymers that include, but are not limited to, inomomeric copolymers or terpolymers of ethylene and unsaturated monocarboxylic acids; vinyl resins; polyolefins; polyurethanes; polyureas; polyamides; acrylic resins and blends thereof; block copolymers; copoly(ether-amide); polyphenylene oxide resins and blends thereof; thermoplastic polyesters; blends and alloys; and mixtures thereof.
Conventional additives that can be incorporated into the golf ball cover of the invention include, without limitation, antioxidants; catalysts; colorants including pigments and dyes; hindered amine light stabilizers; optical brighteners; UV absorbers; fillers; plasticizers; surfactants; viscosity modifiers; compatibility agents; dispersing agents; foaming agents; reinforcement agents; release agents; and mixtures thereof. The cover may be a single layer or multiple layers having at least one inner cover layer and an outer cover layer.
The present invention is further directed to a golf ball having a core, a cover, and at least one intermediate layer disposed between the core and the cover, wherein at least one stable free radical is present in the core, the intermediate layer, or both. Suitable materials for the intermediate layer include natural rubbers; balata; gutta-percha; cis-polybutadienes; trans-polybutadienes; synthetic polyisoprenes; polyoctenamers; styrene-propylene-diene rubbers; metallocene rubbers; styrene-butadiene rubbers; ethylene-propylenes; chloroprene rubbers; acrylonitrile rubbers; acrylonitrile-butadiene rubbers; styrene-ethylene block copolymers; maleic anhydride or succinate modified metallocene catalyzed ethylene copolymers; polypropylene resins; ionomer resins; polyamides; polyesters; urethanes; polyureas; chlorinated polyethylenes; polysulfide rubbers; flurocarbons; or a mixture thereof.
In the presence of the stable free radical, the core and the intermediate layer can be co-cured simultaneously. The core may further have a center and at least one outer core layer. Preferably, the intermediate layer has a thickness of about 0.01 inches to about 0.40 inches, and the cover has a thickness between about 0.01 inches and about 0.10 inches.
As used herein, the term xe2x80x9cminimum torquexe2x80x9d (ML) is the torque of an elastomer measured prior to undergoing a crosslink reaction using a rheometer.
As used herein, the term xe2x80x9cmaximum torquexe2x80x9d (S""max or MH) is the torque of an elastomer measured at any point of time during a crosslink reaction or at the completion of a crosslink reaction using a rheometer.
As used herein, the term xe2x80x9cdelta torque,xe2x80x9d also known as extent of crosslinking, is the difference between the maximum torque MH and the minimum torque ML.
As used herein, the term xe2x80x9cscorch timexe2x80x9d (TS2), also known as scorch safety, is the time to 2% of delta torque above the minimum torque.
As used herein, the term xe2x80x9coptimum cure timexe2x80x9d (TC90), also known as optimum crosslink time, is the time to 90% of the delta torque above the minimum torque.
As used herein, the term xe2x80x9c(meth)acrylicxe2x80x9d includes both methacrylic and acrylic.
As used herein, the term xe2x80x9cfillerxe2x80x9d includes any compound or composition that can be used to vary density, specific gravity, tear strength, rheological and mixing properties, and other properties of the subject golf ball compositions.
The term xe2x80x9cabout,xe2x80x9d as used herein in connection with one or more numbers or numerical ranges, should be understood to refer to all such numbers, including all numbers in a range.